1. Field of the Invention
The present invention relates to a fuel tank, a control system and a method of controlling the fuel tank, in particular a fuel tank for a fuel-cell system.
2. Discussion of the Prior Art
Fuel cells in which hydrogen with oxygen is catalytically converted to water, electric energy being released in the process, are becoming increasingly important for a wide range of different applications. Electric energy is available directly without having to be obtained indirectly via a fuel-driven engine and a power generator. In addition, the reaction between hydrogen and oxygen merely releases water, and no pollutants, with the result that the fuel-cell process is particularly well-suited for applications in which no pollutants should be released. Current application areas include space travel, automotive engineering and mobile power supply. Ever-increasing legal requirements for a reduction in the emission of pollutants mean that fuel-cell technology is of increasing interest for automotive application.
At present, in particular, the use of two technologies are considered for providing the necessary hydrogen. One of these is the direct use of molecular hydrogen which is kept under pressure or in a cooled state in specific fuel tanks and is fed to the fuel cell via a feed means. The other technology is the use-of hydrocarbons which release.the necessary hydrogen in a first stage or during the hydrogen/oxygen catalysis. Examples of such hydrocarbons are methane (CH4) or methanol (H3Cxe2x80x94OH) since these have the highest proportion of hydrogen per carbon atom. Methanol reacts, under suitable catalytic conditions, with water to release hydrogen:
H3Cxe2x80x94OH+H2Oxe2x86x92CO2+3H2
As can be seen, the yields of hydrogen are particularly high when methanol is used. For this catalytic step, it is necessary to use process water, which has to be fed to the methanol before the catalysis. A proportion by volume of methanol to water of 60 to 40% (resistance to freezing to xe2x88x9240xc2x0 C.) is sought here. An excess of water does not adversely affect the reforming of methanol to give hydrogen. A lack of water, in contrast, may result in carbon monoxide being produced rather than carbon dioxide, the carbon monoxide, in turn, adversely affecting the actual fuel-cell reaction between hydrogen and oxygen to contaminate the catalyst.
Since water is produced during the reaction of hydrogen with oxygen (2H2xe2x86x92O2xe2x86x922H2O) in the fuel cell, it is recommended to use this water for mixing with methanol, so that there is no need for any voluminous water tanks. When the process has started, on the other hand, no water is yet available since it first has to be obtained by catalysis. It is thus necessary to keep a certain supply of water by which the process can be started. In the case of mobile applications of fuel cells, for example, their use in motor vehicles, it has to be taken into account that the process water has to be available, and thus must not freeze, even at temperatures as low as xe2x88x9240xc2x0 C. This is expediently achieved in that the water is mixed immediately after it has been formed, in the fuel cell, with its later reactant methanol, and is then stored on an intermediate basis in an intermediate store. The problem with this, however, has been that this intermediate store, in turn, requires a dedicated pump in order to feed the water/methanol mixture to the process during start-up of the latter. A further pump increases the costs and is a potential source of malfunctioning.
The object of the present invention is thus to provide a fuel tank which can supply fuel to fuel-consuming systems, at least over a short period of time, without a pump. Furthermore, a control system and a method of controlling a fuel tank as well as a use of the fuel tank are also to be provided.
The invention is also geared to the use of the fuel tank according to the invention in a fuel-cell system.
The invention provides a fuel tank in which the fuel can be forced out of the tank without a pump, with the aid of a mechanism arranged in or on the fuel tank.
Accordingly, the invention is first of all geared to a fuel tank which is connected to a feed line and a discharge line and has a fuel cavity of variable size, and to a means for compressing the fuel cavity in dependence on the internal pressure of the latter.
The fuel cavity is preferably formed in a hollow body which is designed as a cylinder or prism and in which there is arranged an intermediate wall which can be displaced in a sealed manner along the inner wall of the hollow body by means of a spring. In this case, the fuel cavity is bounded, for example, by a cylindrical inner wall of the fuel tank, by a circular end surface of the fuel tank and by a circular, displaceable intermediate wall. The intermediate wall is forced essentially in the direction of the fuel cavity by a spring and has a form fit with the cylindrical inner wall. The fuel can be forced out of the fuel cavity by virtue of the pressure of the spring on the intermediate wall.
In an alternative, particularly preferred embodiment, the fuel cavity is bounded by an inner wall of the fuel tank and by an intermediate wall designed as an elastic membrane. The inherent elasticity of the membrane generates the necessary pressure in order for it to be possible for the fuel to be forced out of the fuel cavity.
Furthermore, the fuel tank has a separate feed means for feeding fuel into the fuel cavity and a separate discharge means for discharging fuel from the fuel cavity. However, it is also possible to provide just one line to the fuel cavity, via which the fuel is both fed and discharged.
It is possible to arrange in the feed means a feed valve, which may also be designed as a non-return valve. Likewise, it is possible to arrange in the discharge means a discharge valve, it being possible for the discharge valve to be, at the same time, a pressure reducer. It is preferable to use, as the discharge valve, an electrically actuated valve which, in the deenergized state, is closed.
In a further aspect, the invention is geared to a control system for the fuel tank, with a feed valve in a feed means and a discharge valve in a discharge means. The fuel tank is arranged in a fuel-cell system. The control system controls the fuel tank, at least in dependence on the following operating states, as follows:
A) Fuel-cell system is switched on: discharge valve is opened, feed valve is kept closed;
B) Fuel-cell system is in operation: discharge valve and feed valve are kept open;
C) Fuel-cell system is switched off: discharge valve is closed and feed valve is kept open; and
D) Fuel-cell system is off, fuel cavity is full: discharge valve and feed valve are kept closed.
The invention also comprises a method of controlling the fuel tank, with a feed valve in a feed means and a discharge valve in a discharge means, the fuel tank being arranged in a fuel-cell system, and the fuel tank being controlled, at least in dependence on the following operating states, as follows:
A) Fuel-cell-system is switched on: discharge valve is opened, feed valve is kept closed;
B) Fuel-cell system is in operation: discharge valve and feed valve are kept open;
C) Fuel-cell system is switched off: discharge valve is closed and feed valve is kept open;and
D) Fuel-cell system is off, fuel cavity is full: discharge valve and feed valve are kept closed.
Finally, the invention is also geared to the use of the fuel tank according to the invention in a fuel-cell system, in particular in a fuel-cell system for vehicle propulsion, although this use is not to be taken as limiting. Rather, the fuel tank according to the invention may also advantageously be used in other propulsion and energy-generating systems, provided that fuel supply which is not driven by a pump is desirable. In aircraft for general air travel, use is usually made, for example, as an emergency system, of an electric fuel pump which can still supply the engine with fuel even if the main pump is defective. However, this pump is dependent on the functional capacity of the electrical system. Using the fuel tank according to the invention of a suitable size, however, would make it possible to realize a purely mechanically operating backup system which would keep the engine functional at least for a period of a few minutes, for example as an emergency landing is carried out.
For use in a fuel-cell system, the maximum extent of the volume of the variable fuel cavity is advantageously sufficient to ensure the fuel supply for the heating-up phase of the fuel-cell system. A possible size for the fuel cavity is between 0.1 and 10 l, preferably between 0.1 and 5 l, and particularly preferably between 0.2 and 1 1. Larger fuel cavities are also conceivable if there is a corresponding fuel requirement.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.